When a specimen is observed using an electron microscope (i.e., an electron microscope main body, which will be generally referred to simply as "electron microscope", hereinafter), it is general practice to pick up a magnified image of a specimen formed on a fluorescent panel in the microscope by a TV camera. This is effected for the purpose of converting information concerning the specimen into electric signals for effecting picture processing. In addition, this imaging operation is carried out for the purpose of adjusting the focus of the microscope in the case of photographically recording a magnified image which is discernible to the naked eye because the irradiation dose is minimized in order to suppress the generation of damage to the specimen.
When a magnified image obtained by an electron microscope is picked up by a TV camera for the purpose of effecting focusing, it is conventional practice to experientially make a judgement by examining the extent to which the magnified image is out of focus, or the change of a Fresnel fringe of the magnified image, i.e., a kind of interference fringe which is formed by a beam of electrons reflected from an edge portion of the specimen. However, the Fresnel fringe is not necessarily formed by any kind of specimen but appears depending upon the kind of specimen. In addition, it is difficult to make a judgement as to whether or not the microscope is in focus from the extent to which the magnified image is out of focus. When an out-of-focus image is photographically recorded, the extent to which the image is out of focus is enhanced. Therefore, if the focus of the microscope is not satifactorily adjusted when a magnified image which is picked up by a TV camera is observed, an image which is more out of focus is recorded on a picture.
A wobbler device has heretofore been employed in an electron microscope for the purpose of facilitating the adjustment of focus. The wobbler device is based on the principle that, as the incident angle at which an electron beam is applied to a specimen increases, the focal depth of an objective lens becomes smaller. More specifically, the wobbler device changes the direction in which an electron beam enters a specimen so as to increase the apparent incident angle, thereby enlarging the aperture of the electron beam with respect to the incident direction of the electron beam, and thus facilitating the focus adjustment by means of the objective lens. The wobbler device has already been disclosed in, e.g., the specification of Japanese Patent Laid-Open No. 30154/1977.
However, when an electron microscope provided with a wobbler device and a TV camera of the type described above are combined together, the following problems may arise. Namely, the TV camera successively scans over the imaging surface to obtain a picture signal. Therefore, when the wobbler device is operated during the imaging operation by the TV camera in order to change the incident direction of the electron beam, if the speed at which the deflection angles are changed over by the wobbler device is excessively high, images may undesirably overlap or lie one upon another while one frame is being replaced by another, which means that it is difficult to effect the focus adjustment.